The Prymnesiophyceae are characterized
partly by sub-light-microscopic features, including plastids surrounded by
four layers of membrane indicating a history of successive “packaging” by
endocytosis.Thylakoids are stacked in
triplets, chlorophyll is both a and c, and the storage reserve is
chrysolaminarin.Other classes
(Eustigmatophyceae, Raphidiophyceae, and Tribophyceae) share all these features.

The name "Prymnesiophyceae" was established by Hibberd (1976) to satisfy the International Code of Botanical Nomenclature by using the root of a genus (with the use of Prymnesium in this case). The previous name "Haptophyceae" (Christensen 1962) was based on the presence of an appendage unique to this protistan class,
located between the flagella that superficially resembles them, the
haptonema that has no homologue outside the class.Various species have
differing lengths and flexibilities of haptonemas, and their function also
varies from grasping objects to none observed.

Most species are marine (~300), fewer than 10
are in freshwater.Freshwater habitats
include lakes with low conductance.Blooms have occurred after loading of organic solutes (human sewage)
in a New Hampshire lake.

Some genera including the coccolithophores and Phaeocystis are strictly marine
plankters.

Hibberd, D.J. 1976.The ultrastructure and taxonomy of the
Chrysophyceae and Prymnesiophyceae (Prymnesiophyceae):A survey with some new observations on the
ultrastructure of the Chrysophyceae.Bot. J. Linnean Soc. 72:55-80.